This invention relates generally to calibrating digital cameras.
Digital cameras may use a solid state sensor as the imaging array. Typical solid state sensors include charged coupled devices and active pixel sensors. Cameras using solid state sensors are subject to photo response nonuniformity (PRNU) and defective pixel maps. Photo response non-uniformity is systematic and is largely due to small variations in device processing across a particular imaging array. The non-uniformity introduces noise to the resulting picture. However, since the noise is systematic, it can be cancelled out following an appropriate calibration procedure.
Similarly, the imaging array may have defective pixels or elements and if the number of defective elements is not too great, the imaging array may still be useable. For example, calibration techniques can be utilized to overcome the effect of a relatively small number of defective elements in the imaging array.
There are a number of problems with calibrating digital cameras at the factory. For one thing, the quality of the calibration technique is to some degree a function of how much time is used to accomplish the calibration. The ideal calibration involves analyzing a large number of images and extracting the non-uniformity noise from other noise sources so that the total system noise is reduced. Thus, the more images that are analyzed the greater the likelihood that camera noise and photon shot noise may be eliminated so that photo response non-uniformity may be isolated and calibrated out. Of course, expending more time during factory calibration, results in increasing expense to the user.
In addition, it is difficult to convey the PRNU cancellation information to the user since the calibration data consists of a relatively large file of noise information. Generally, noise data cannot be readily compressed and therefore it would be necessary for the factory to convey a relatively large file in (or with) the camera. This could result in ineffective use of the camera's memory and awkward user startup. Particularly in view of the fact that many digital cameras have relatively little onboard memory, it is not desirable to provide an extensive file of noise information in cameras with limited memory.
Of course, it is also possible that the user could attempt to self-calibrate the camera. One problem the user must face is that calibration techniques generally require a flat field light source. Generally, inexpensive flat field sources are not available. The absence of a flat field light source and the need for some sophistication, would likely limit the number of users who could effectively self-calibrate digital cameras.
Another issue impacting digital camera cost is the necessity for a relatively high percentage of the elements making up the imaging array to be properly functional. If a relatively small number of the individual elements making up the array are defective, this may not be noticeable. Defective array elements may be compensated for by the calibration process. The higher the required ratio of good array elements to the total number of elements in a given array, the higher the resulting manufacturing cost.
Thus, there is a continuing need to enable cost effective calibration of digital cameras. There is also a need for flat field calibration techniques that are applicable to use in the field. Similarly, there is a continuing need to enable manufacturers to decrease the required ratio of good elements to total elements in an imaging array while still producing fully adequate image quality in the resulting stored images.